The present invention relates to ram assemblies of the type used in the oil and gas industry and, more particularly, to a shear ram assembly which provides more reliable sealing between an upper blade and a lower blade.
Ram assemblies referred to as blowout preventers (xe2x80x9cBOPsxe2x80x9d) are commonly used in the oil and gas industry while exploring for and extracting hydrocarbons, such as oil and gas. One or more commonly two ram assemblies are used in series to cut off the flow from the well in an emergency. Some ram assemblies are intended to seal off an annulus surrounding a tubular, while other ram assemblies have the ability to both severe the tubular and seal in the well. Blades fixed to opposing shear rams may thus be driven together to completely shear the tubular. The lower portion of the severed tubular typically remains in the well beneath the shear rams, and the upper severed portion may be removed. The cut edge of the upper and lower tubular portions are sometimes referred to, respectively, as the upper and lower xe2x80x9cfish,xe2x80x9d and xe2x80x9cfishing toolsxe2x80x9d are used to retrieve the lower fish. A sealing element acting between the upper and lower shear blades or between each blade and the shear ram housing prevent fluid from escaping through the closed shear ram assembly.
Blade configurations and corresponding sealing elements for shear ram assemblies are numerous in the prior art. One general type of seal may be described as a face seal, in which a face or a leading edge of one or both rams comes to rest forcefully against a seal element after shearing. U.S. Pat. No. 3,561,526 describes a shear ram assembly which is intended to reduce the likelihood of damage to the sealing elements as a result of the shearing process. After the two rams sever the tubular, the rams continue to slide past one another until each ram presses against an opposing seal element. This creates two vertically spaced pressure seals, with a greater combined likelihood of reliable sealing than might be possible with just one seal.
In other shear rams, the sealing element is instead carried in the sliding face of the upper blade to seal between the blades after shearing. Although the cutting edge of the blade passes over the sealing element, which can damage the seal, this arrangement is preferred for many applications since the seal does not depend on a known final position for the blades. Numerous patents propose minimizing the risk of seal damage with this type of shear ram assembly. U.S. Pat. No. 4,540,046 is one example of a shear ram assembly comprising a seal actuator which actuates the seal into compressive engagement with the opposing blade only after the cutting edge of the blade has passed over the seal. U.S. Pat. No. 5,713,581 discloses a seal configuration designed to resist the damaging rollover of the seal element that may occur when one blade slides over the seal element. U.S. Pat. No. 4,347,898 discloses a shear ram assembly intended to withstand high forces tending to separate the blades without damage to the seal. U.S. Pat. No. 5,515,916 discloses shear rams in which a cross seal strip on one blade face is constructed and arranged to avoid being damaged by the other blade. Avoiding damage to the sealing element is clearly a fundamental concern for shear ram assemblies that seal between the blades.
Prior art shear ram assemblies thus include a seal element carried in the lower face of the upper blade. U.S. Pat. Nos. 4,540,046, 5,713,581, and 4,537,250 are examples of this type of configuration. One reason for this preference is the lower surface of the upper blade and thus the seal element is protected by the upper blade from dirt and debris. Furthermore, fluid pressure from the well beneath the rams helps energize the seal in those assemblies having an actuator bar, since fluid pressure from below acts on the actuator bar to further activate the seal.
In addition to protecting the sealing element, another primary concern associated with shear ram assemblies is the time and expense involved with maintenance. Millions of dollars are routinely spent in the operation of a wellhead, and downtime results in loss of revenues and profitability. The oil and gas industry is therefore interested in equipment that is less expensive and time consuming to operate. U.S. Pat. No. 5,360,061 discloses shearing rams which shear a tubing string in such a manner that a separate trip may not be required to prepare the upper fish left in the well bore prior to lowering an overshot to engage the upper end of the sheared tubing string. U.S. Pat. No. 5,400,857 discloses a shear ram assembly intended to minimize equipment costs and operating expenses. Other patents of interest include U.S. Pat. Nos. 4,341,264, 4,240,503, and 5,906,375.
The present invention surpasses the prior art, offering a shear ram assembly with an improved sealing system. The sealing system avoids the drawbacks of the prior art, particularly for those shear ram assemblies having a sealing element between the sliding blades.
The present invention relates to improvements in a shear ram assembly, which is commonly part of a BOP which contains an upper shear ram assembly and a lower pipe ram assembly. The tool joint of the oilfield tubular is conventionally hung off on the lower pipe rams once the upper shear ram assembly is actuated to shear the tubular. More particularly, the present invention relates to improvements in the location of the sealing system which seals between an upper shear blade and a lower shear blade of the shear ram assembly.
The shear ram assembly may thus be actuated to shear a tubular extending into a well and seal fluid pressure within the well. The assembly comprises a first ram axially moveable along a first axis for moving an upper blade, and an opposing second ram axially moveable along a second axis for moving a lower blade, with the first and second axes preferably being substantially coaxial and perpendicular to a centerline of the shear ram assembly, which coincides with a centerline of the tubular being sheared. The upper blade and the lower blade are each slidably moveable toward the centerline of the tubular, and at least one and optionally both of the upper blade and the lower blade include a shearing edge for shearing the tubular. An upper planar surface of the lower blade thus passes closely below a lower planar surface of the upper blade as the blades move toward each other in response to actuation of the first and second rams to shear the tubular. The sealing system is positioned substantially within a recess in the upper surface of the lower blade, and includes an elastomeric seal for sealing engagement with the lower planar surface of the upper blade and a rigid actuator moveable relative to the lower blade to energize the elastomeric seal.
In a preferred embodiment, the actuator includes an energization ramp for engagement with the upper blade as the blades move toward each other, such that the upper blade urges the actuator downward to compress the elastomeric seal, thereby energizing the elastomeric seal. The actuator preferably comprises an elongate actuator bar having a bar axis generally perpendicular to the centerline of the shear ram assembly. A retainer bar supported on the lower blade overlaps a portion of the actuator bar to restrict upward movement of the actuator bar.
It is an object of the present invention to provide an improved shear ram assembly wherein the sealing system is positioned substantially within a recess in the upper surface of the lower blade, such that the top edge of a lower fish does not contact and damage the elastomeric seal when the shear ram assembly is subsequently opened, thereby extending the useful life of the sealing system compared to a shear ram assembly wherein the elastomeric seal is provided in a lower surface of the upper blade.
It is a feature of the present invention that the lower surface of the upper blade may be hardened, preferably by a laser hardening process, to resist damage by the sheared tubular after the shearing operation, specifically when the shear ram assembly is opened.
A further feature of the invention is that the sealing system includes a leading anti-extrusion insert and a trailing anti-extrusion insert each adjacent an upper surface of the elastomeric seal to prevent pressurized extrusion of the elastomeric seal into a gap between the upper surface of the lower blade and the lower surface of the upper blade.
A related feature of the invention is that the upper blade includes a leading edge surface sloping upward from the lower surface of the upper blade to an upper blade edge for initially engaging the sealing system, thereby minimizing the likelihood of the upper blades damaging the sealing system.